The present invention relates to material testing. More Particularly, the present invention relates to the testing of adhesion properties of thin film materials.
The microelectronics industry is ever increasing the density of components to meet cost and performance demands of the consumer. Reliability and service life are of concern to both the industry and customers. One technology trying to meet those demands is thin film technology. The industry is researching the application of various new materials to meet the low dielectric constant requirement for improved back-end of line interconnect performance.
Successful application of the new materials requires maintaining mechanical integrity, including fracture resistance, throughout the multilayering processes. The adhesion of grown and deposited films must be excellent to ensure good reliability and service life of the resulting products. In most cases, adhesion is strongly affected by the cleanliness of the substrate. Contamination of interfaces results in reduced adhesion, as does an adsorbed gas layer. Substrate surface roughness can promote adhesion, but may also result in coating defects.
Adhesion of thin films and coatings to various materials is of importance when they are subjected to various conditions or processes. Because electronic devices are fabricated from a variety of materials using silicon substrates and various applied films, stresses develop between layers as a result of expansion mismatches between the laminations. Poor adhesion represents a potential reliability problem. If films lift from the substrate, device failure can result.
The adhesion of thin films is particularly important in the microelectonics industry where devices are subjected harsh conditions. With the industry drive toward smaller, higher speed devices, materials for back-end of line processes are needed to reduce resistance-capacitance time delay in next generation integrated circuits. Hence, process integration now involves new thin film materials such as low k, copper and other novel materials.
Several materials are prone to delaminate or exhibit detrimental physical and material changes upon heating and cooling. Problems are compounded by the requirements of multiple layers of coatings or thin films. Consequently manufacturers are requiring greater adhesion specifications. Failing to meet these greater specifications can preclude the manufacturer""s product from entering the marketplace. Thus, data on the mechanical reliability of these new materials is critical. However, since such data is not available, sufficient test data needs to be developed to aid in the prediction of a product""s use limit or lifetime and reliability.
Adhesive failure can be predicted when applied energy exceeds a critical fracture property of a union. The demand in designing and measuring adhesion is to establish the characteristics of both the applied energies and the critical performance properties. Performance properties vary with a myriad of processing and environmental conditions, hence, any test developed to measure these properties must be capable of simulating the same processing conditions.
Of great concern is adhesive failure due to large thermal stresses developing during processing. To be able to predict the behavior of a design, failure data must be available to compare to known stress fields.
Early attempts to measure adhesion included the use of the tape test and a method of abrasion. The tape method consisted of pressing a piece of adhesive tape to the film. The tape is then pulled off the film either leaving the film intact, removed in whole or in part, or remaining on the substrate. This method is qualitative only, and if the film remains on the substrate, it provides no quantitative data as to the magnitude of the adhesion forces. Failure of the tape test implies that the film is unsuitable for device fabrication.
The abrasion or scratch test method results depend on the film hardness as well as on its adhesion. All of these conventional tests for adhesion are qualitative at best and do not accurately model the fracture mechanism.
The modified Edge Liftoff Test has been developed and applied to testing multilevel coatings on a rigid substrate. This test is applicable to testing the adhesiveness of multilayers of microelectronic structures in that it allows testing of samples constructed with standard back-end of line processes.
A thick coating of epoxy is applied to a multilayer device. Failure of the adhesive forces is caused by the stored strain energy in the thick epoxy layer exceeding the critical adhesion energy of the weakest component interface as the test sample is cooled. Advantages of this type of testing are simplicity and resultant true fracture energies of the system. Reliability of the device can be quickly assessed by comparing the measured fracture energies to the calculated applied fracture energies from finite element analyses. However, this system generally requires a human observer to continually peer through glass plates to monitor delamination of the films.
In view of the above, what is needed is a modified edge lift test system capable of providing consistent data of the adhesion characteristics of films and coatings. The system should be capable of sealed automatic temperature processing, including computer controlled delamination detection and temperature control.
To address current industry needs, the invention offers a modified edge lift test system which includes a sealed automatic temperature processing type chamber that operates in a computer controlled heating and cooling mode, and a computer processor with delamination detecting software. The invention""s ability to provide consistent data of the adhesion characteristics of old and new films and coatings can provide valuable processing data to the industry and speed up the manufacturing process. This invention is particularly applicable to industries such as semiconductor manufacturing where the search for new thin film materials and coatings is ongoing.
The invention provides for an improved system for quantitative and qualitative testing of the adhesion characteristics of thin film materials and coatings during thermal processing. In one embodiment, an apparatus of the present invention includes an atmosphere sealable chamber set within a metal housing. An optical window made of a transparent temperature stable material such as Plexiglas, glass or quartz mounted in a wall of the chamber to view the samples being tested. A sample tray holder, capable of holding multiple samples, inserted though a side wall of the chamber and lockable into position. A camera is mounted on top of the optical window to replace a human observer in monitoring the testing process. Lighting is mounted adjacent to the improved optical window to provide a sufficient amount of light to illuminate the samples being tested for observation and recordation. A computer processor, with image processing software, is connected to the testing apparatus and the camera for collection and analysis of temperature and visual data gathered during the testing process.
In another embodiment, a sample tray is inserted into the chamber through a slot in the wall of the chamber and sealed with clamps. In yet a further embodiment, a single viewing window made of a solid temperature stable material is used to address the problems of shadows, ghost images, double reflection, and frost.
In one aspect of the invention, two lamps, one mounted on two different sides of the viewing window are utilized to provide a constant and sufficient amount of light for optical viewing and recording by a camera mounted on top of the viewing window. The lamps provide a ghost free, shadow free, viewing and recording of sample images.
Advantageously, the present invention provides more efficient thermal transfer by using an improved heat exchanger where the heating/cooling surface has raised posts. Moreover, the present invention uses an automated gas and liquid nitrogen mixer assembly. Through the use of gages or reducers and valve configuration, the computer controls the opening or closing of valves to set the flow of gas and liquid nitrogen into proper ratios prior to introduction into the chamber for either a specified temperature range or time period.
Another advantage of the present invention is the use of a digital camera to record an image change in the place of a human operator and computer imaging software which captures and records debonding of samples and the temperatures at which debonding occurs and then calculates the debonding energies. In addition, the present invention provides complete automation and control of the entire process by computer, from heating through cooling and calculation of debonding energies, with graphing of data in a visual chart for the user.